Optical wavelength division multiplexed transmission systems currently operate at 10 Gbit/s and below. However, the growing demand for capacity in terrestrial transmission systems and the prevailing desire to save or at least conserve current wavelength resources and simplify network management make increasing the channel rate to around 40 Gbit/s an attractive proposition. At increased bit rate a number of effect that limit system performance become more significant. These effects include chromatic dispersion, non-linearity, polarization mode dispersion (PMD) and amplified spontaneous emission (ASE). This attendant increase in noise requires an increase in channel bit rate to be accompanied by a linear increase in power to return the same signal-to-noise ratio (SNR). However, the non-linear effects become more severe as the power is increased. The optimum signal power is thus generally viewed as a trade-off between non-linearity and SNR. One of the dominating non-linear effects for high bit rates of 40 Gbit/s is that due to the interaction of overlapping pulses. At these higher bit rates the fibre dispersion causes the pulses to spread rapidly and while the average dispersion can be limited to close to zero by using dispersion-compensating fibre (DCF), the pulse energy will be spread over many bit-slots during most of the transmission. Non-linear pulse-to-pulse interactions take the form of intra-channel four-wave mixing (IFWM), wherein two or more wavelengths within the same channel interact to create a new wavelength causing amplitude variations, and intra-channel cross-phase modulation (IXPM), wherein several different wavelengths can cause each other to spread out resulting in timing jitter.
In most of the current optical WDM systems operating at 10 Gbit/s and below, the modulation format used is non-return-to-zero (NRZ). However, it has been suggested that the return-to-zero (RZ) modulation format is better suited for higher bit rate applications. While the RZ format requires more complex transmitters it has a higher tolerance to non-linear effects. Several schemes have been proposed for improve transmission performance by increasing the non-linear tolerance using phase modulation. U.S. Pat. No. 5,526,162 suggests increasing the non-linear tolerance by broadening the signal spectrum. This is achieved by modulating the phase of a NRZ optical data signal synchronously with the bit rate. U.S. Pat. No. 5,946,119 and U.S. Pat. No. 6,005,702 suggest the same form of modulation applied to a RZ optical data signal. When the phase of a RZ signal is modulated synchronously with the bit rate by a sinusoidal waveform the resulting modulation format is referred to as chirped RZ (CRZ). EP 1 059 758 proposes an alternative scheme, wherein a phase modulation is introduced that is synchronised with half the bit rate. This is referred to as carrier-suppressed RZ (CS-RZ) modulation and is achieved by shifting the phase between RZ pulses in adjacent bit slots by an odd integer of π. The resulting scheme has a higher non-linear tolerance than conventional RZ or NRZ. A further phase modulation format is described in R. Ohira et al., “Novel RZ signal format with alternate-chirp for suppression of non-linear degradation in 40 Gbit/s based WDM”, Proceedings of OFC 2001, paper WM2, (2001). In this format a signal is phase modulated using a sinusoidal signal at a first frequency, filtered using a narrow band filter to extract the carrier and first double-side-band components and then data encoded with a RZ modulator at a frequency twice the phase modulation frequency. The resulting modulation, termed alternate chirp, has a non-linear tolerance similar to chirped RZ but with the narrow spectrum of CS-RZ.
Of the known modulation schemes described above chirped return-to-zero modulation (CRZ) provides the best non-linear tolerance. However, this is at the expense of a broadened spectrum, which brings with it the problem of a lower dispersion tolerance and reduced spectral efficiency.
In view of these known schemes an object of the present invention is to propose an improved modulation format capable of overcoming the problems associated with the prior art.
A particular object of the present invention is to propose a modulation format that is capable of effectively combating the influence of non-linear pulse-to-pulse interactions, which is the dominating non-linear effect in optical fiber transmission at bit rates of 40 Gbit/s and above.